Development of the Jugular Lymph Sacs. 207 
us, agree in every detail, and even the same embryo often shows 
variation and different degrees of development on opposite sides. 
In the description of the individual series these special differ- 
ences will be taken up in detail, but prior to their consideration it 
is desirable to establish the general conditions and the genetic 
principles involved in veno-lymphatic development. 
A great part of the observed variability depends upon the fact 
that the jugular lymph sac of the later stages (fig. 17) is preceded 
by the formation of three primary veno-lymphatic plexuses 
(fig. 12). Subsequently these unite to form the complete jugular 
lymph sac (figs. 12 to 17). 
In this process two of these plexuses first unite to form a common 
ventral division of the sac, which then.fuses with the third plexus 
placed dorsally, to constitute the common jugular lymph sac of 
the later embryonic stages and of the adult. 
We have consequently adopted the following descriptive ter- 
minology: 
I. Ventral Veno-Lymphatic Plexus. 
A. Cephalic or Anterior Division. 
B. Caudal or Posterior Division. 
II. Dorsal Veno-Lymphatic Plexus. 
These three veno-lymphatic plexuses are of primary importance 
and represent a developmental stage intermediate between the 
purely venous and early veno-lymphatic conditions, and the sub- 
sequent period, in which the veno-lymphaties have fused to form 
a common sac. (cf. figs. 12 and 46). 
For clearness of description we will divide the account of the 
development of the jugular lymph sac into two periods: 
I. From the Development of the Early Venous Stage up 
to the Establishment of the Three Primary Veno- 
Lymphatic Plexuses. 
A. Development of the Ventral Veno-Lymphatic Plexus. 
1. Cephalic or Anterior Division. 
2. Caudal or Posterior Division. 
B. Development of the Dorsal Veno-lymphatic Plexus. 
